Charging the power dock with varying power source

I have a plan to connect a 5v solar panel to my power dock with the intention of charging the battery during the day so it can continue to run at night. With a large enough panel and a 3000mah battery this shouldn't be any problem, but before I go spending a few dozen dollars on what I need I want to make sure the power dock itself can do this.
How does it behave when the charging amps drop below the usage amps? As the sun goes down, will it automatically start siphoning off the battery? Or will I need to implement a cut-off switch to completely break the line when it can't keep up with the need? I'm reasonably certain it's the former but hoping someone can confirm.
Thanks!